Radomes

ABSTRACT

A radome especially suitable for circularly polarized waves and antennas is described. The radome has an anisotropic core made up of parallel dielectric strips which may be in the form of dielectric rings which extend circumferentially around the wall of dielectric material which makes up the radome. Alternatively, the strips may extend longitudinally along the wall.

)(Rv 39576q58l [72] Inventors Gus P. Tricoles; 2,744,042 5/1956 Pace(343/872UX) EugeneL. Rope,SanDiego, Calif. 2,929,581 3/1960 Johnson,Jr....(343/872RUX) [21] Appl. No. 752,948 3,175,220 3/1965 Schetne 343/872[22] Filed Aug. 15,1968 OTHER REFERENCES 3 g m 8 C .R. M. Kubow et al.,AVIATION AGE, Feb. 1958, pp 74 l 3] 78. Copy in Scientific Library, TL501.A83. 343 872R Primary Examiner-Rodney D. Bennett, Jr. [54] RADOMESAssistant ExaminerRichard E. Berger 12 Claims, 5 Drawing Figs.Attorney-Martin Lu Kacher [52] U.S.Cl 343/872,

343/756 [51] hit. Cl ABSTRACT; A radome especially uitable forcircularly Field Of Search olarized waves and antennas is described Theradome has an 872 anisotropic core made up of parallel dielectric stripswhich may be in the form of dielectric rings which extend circum- [56]References cued ferentially around the wall of dielectric material whichmakes UNITED STATES PATENTS up the radome. Alternatively, the strips mayextend longitu- 3,444,550 5/1969 Leitner 343/872 dinally along the wall.

l4 l l A I I I 1 11 u u 1/ H PATENTEDAPRZ'HQYI 3576.581

SHEET 1 0F 2 Fig.

'9). 605 F! TRICOLES EUGENE L. ROPE BY W. ATTY F lg. 2

RADOMES The present invention relates to radomes and particularly toradomes having improved transmittance for circularly polarized waves.

The invention is especially suitable for use in radomes which aredesigned to pass waves which are incident thereon at fairly high anglesof incidence (by angle of incidence is meant the angle between a ray anda line perpendicular to the wall of the radome on which the ray isincident). The invention is also especially suitable for use in a radomefor circularly polarized waves.

With the advent of needle-nose aircraft, it becomes necessary that theradome which forms the nose of the aircraft be capable of transmittingwaves from an antenna which lies within the nose with maximum powertransmission, even though the angle of incidence of such waves to thewalls of the radome is fairly high. The problem becomes more acute incases where circularly polarized waves are to be transmitted to theradome. Known radome constructions provide different transmissioncharacteristics, particularly phase delays, between the perpendicularand the parallel polarization components which make up the circularlypolarized wave. Thus, pattern distortion and internal reflections resultwhich degrade the performance of the radar or other microwave systemlocated within the radome.

It is therefore an object of the present invention to provide animproved radome which passes circularly polarized electromagnetic waveswith minimum pattern distortion and internal reflection.

It is a further object of the present invention to provide an improvedradome which has high transmittance even at high angles of incidence ofelectromagnetic waves with respect thereto.

It is a still further object of the present invention to provide animproved radome which is capable of transmitting electromagnetic wavesover a broad band of frequencies (say percent of the center frequency ofsuch waves).

It is a still further object of the present invention to provide animproved radome having improved structural strength as well as improvedelectrical properties.

Briefly described, a radome embodying the invention has a dielectricwall so structured as to have anisotropic electromagnetic wavepropagation characteristics. In other words, the dielectric constant ofthe wall as a result of the anisotropy is higher for parallel than forperpendicular polarization of the electromagnetic radiation. Ananisotropic radome wall in accordance with the invention includes adielectric sheet having dielectric strips mounted thereon. The stripsmay be disposed transversely (e.g. edgewise) to the plane of the wall.The strips may form part of a core of an A-sandwich wherein the skinsare relatively thin (much less than a quarter wavelength of theradiation to be transmitted). In the A-sandwich case, the strips reducethe difference between the phase delay for parallel polarization andthat for perpendicular polarization. The resultant phase difference ofthe entire wall including the core is reduced below that of a radomehaving a hollow core or a skin alone.

In the event that the skin is relatively thick, say of the order of ahalf wavelength of the radiation to be transmitted, the strips may be inthe form of circumferential rings or discs. The invention itself, bothas to its organization and method of operation, as well as additionalobjects and advantages thereof will become more readily apparent from areading of the following description in connection with the accompanyingdrawings in which:

FIG. 1 is an elevational view of a radome embodying the invention, theview showing a portion of the outer skin of the radome broken away toillustrate the construction of the core;

FIG. 2 is a fragmentary view of the core of a radome such as shown inFIG. 1;

FIG. 3 is a sectional view in elevation of a radome in accordance withanother embodiment of the invention;

FIG. 4 is a curve of the power transmittance of the radome shown in FIG.1; and

FIG. 5 is a curve similar to FIG. 4 showing the power transmittance ofthe radome shown in FIG. 3. Referring to FIG. 1, a conical radome isshown having an A-sandwich panel construction. The A-sandwich is madeout of a thin outer skin 10 of dielectric material, such as fiber glasssheets laminated with epoxy plastic. Quartz cloth may also be used. Thethickness of the skin 10 is, desirably, much less than a quarterwavelength at the frequency to be transmitted. Thus, for Ku-bandradiation, a thickness of approximately 0.02 inches may be suitable. Athin inner skin 12 of material, similar to the outer skin and of athickness similar to the thickness of the outer skin, is separated by acore made up of a plurality of thin strips 14 of dielectric material.These strips may be made of material similar to the skins (viz. a fiberglass laminate using an epoxy resin to laminate a pair of fiber glasssheets). The edges of the strips may be cemented to the skin so that thestrips are perpendicular to the skins. The strips are desirably spacedequally from each other. Inasmuch as the spacing is desirably equal,some of the strips are shorter than others in order that the spacing mayremain equal all the way to the apex of the conical surface of theradome. The spacing between the strips is a function of the frequency ofthe radiation to be transmitted. At a frequency of 9.225 GI-lz, aspacing of 0.65 inches was found suitable.

The dielectric constant of the radome is higher for parallel than forperpendicular polarization, particularly at high incidence angles of theradiation to the wall of the radome. By parallel polarization is meantthat the E.-vector of the radiation is parallel to the plane ofincidence, which contains the incident wave normal and the local normalto the surface. For perpendicular polarization, the E-vector is normalto the plane of incidence. In FIG. 1 the strips are approximatelyparallel to the plane of incidence. The strips reduce the differencebetween the phase delays for perpendicular and parallel polarizations.The effect of this reduction, on the transmission of circularlypolarized waves, is explained as follows: 8 is defined as the differencebetween the insertion phase delay for perpendicular polarization andthat for parallel polarization. Briefly 5 equalsI PD I PD jlhe IPD ofthe sandwich made up of the skins l0 and 12 and the core containing thesteps 14 is the sum of the [PD of the core. 8 and the IPD of the skins,6, and 8, That is, for the sandwich If 8 O and 8 0,6 can be reduced bymaking 8 0. This is what the core made up of the steps 14 does. Thepower transmittance for a circularly polarized plane wave for circularpolarization is Where T and T are complex-valved amplitudetransmittances for parallel and perpendicular polarizationsrespectively. Equation 2 shows minimizing 8 increases T In order toenhance the anisotropy of the radome, it is desirable to use a honeycombof fiber glass material in the space between the longitudinal strips 14(see FIG. 2). The cells 16 of the honeycomb are rectangular in shape.The anisotropy favoring the parallel polarized waves is enhanced byhaving the long dimension of the cells 16 substantially parallel to thelongitudinal strips 14. The honeycomb core containing the strips 14 isstrong electrically and mechanically. Accordingly, the radomeconstructed in accordance with the invention may be relatively large insize without the need for additional internal support.

The power transmission of the radome shown in FIGS. 1 and 2 isillustrated in the graph of FIG. 4. The abscissa of the graph iscalibrated in terms of a gimbal angle. By gimbal angle is meant theangle between the-axis of the cone and the axis of an antenna, such asthe horn antenna 20, shown in FIG. 3. When the cone axis and the antennaaxis coincide, the gimbal angle is 0. Small gimbal angles thereforecorrespond to high angles of incidence of the radiation with respect tothe wall of the radome. The dash line represents the radome, includingthe core having longitudinal strips, as shown in FIG. 1. The solid linerepresents the core without the longitudinal strips. In both cases, thecurves were taken with circularly polarized radiation produced by aconical horn antenna, such as the antenna shown in FIG. 3. A significantimprovement in transmittance is apparent from FIG. 4.

Referring to FIG. 3, there is shown a conical radome having a skin madeof dielectric material. The skin is relatively thick as compared to thethin skins or sheets used in the A-sandwich radome construction shown inFIGS. 1 and 2. Specifically, the thickness of the skin or sheets 22 maybe about one-half wavelength at the frequency of the radiation to betransmitted. The material of the conical sheet 22 is desirably of aceramic nature, such as alumina. Supported within the conical sheet onthe inner surface thereof are a series of equally spaced circular discs24. These discs may be made of fiber glass sheets laminated with epoxy.The thickness of the strips may, for example, be about 0.04 inches andtheir spacing may be about 0.65 inches. The thickness of the sheets andthe spacing is a function of the frequency of the radiation to betransmitted. The above given examples of thicknesses and spacings may besuitable for a frequency of approximately 16.0 GI-Iz. The antenna 20which is shown disposed within the radome is designed to propagatecircularly polarized waves. To this end, a plastic wedge 28 of material,such as polystyrene is disposed in the waveguide section of the horn 20and delays the components of the radiation aligned with it so that theE-vector of the radiation rotates circularly. Accordingly, the radiationemitted by the horn 20 is circularly polarized.

The improved results obtained with the radome shown in FIG. 3 areillustrated in FIG. 5. The solid line curved represents thetransmittance of the radome, including the discs 24. The same radome,but without the discs, provides the transmittance shown by the dash linecurve. The power transmittance (lTl is calibrated in normalized form onthe ordinance of the curve. Accordingly, it will be observed that theimprovement in transmittance is very substantial.

From the foregoing description it will be apparent that there has beendescribed different embodiments of a radome which is especially suitablefor the transmission of circularly polarized radiation. Simplificationsin the illustration have been made in order to more concisely andclearly explain the invention. Variations and modifications in theherein described embodiments will undoubtedly suggest themselves tothose skilled in the art. Accordingly, the foregoing description shouldbe taken merely as illustrative and not in any limiting sense.

We claim:

1. A radome comprising:

a. a sheet of dielectric material, and

b. an array of thin strips of dielectric material supported edgewise onsaid sheet with the edges of said strips in contact with said sheet,said strips being arranged in planes spaced from each other, said planesbeing parallel to the plane of incidence of at least one of twopolarizations of radiation which are perpendicular to each other, saidplanes lying transverse to said sheet, said array providing anisotropyfavoring said one polarization of radiation.

2. The invention as set forth in claim 1 wherein said strips areparallel to each other.

3. The invention as set forth in claim 2 wherein said strips each are atleast a quarter wavelength in a direction along the plane of said onepolarization of radiation and are a fraction of said quarter wavelengthin thickness measured from side to side along their supported edges.

4. The invention as set forth in claim 3 wherein the spacing of saidstrips from each other is equal and is from one-half to 1 /2 of awavelength of the frequency of the radiation to be transmitted throughsaid radome.

5. The invention as set forth in claim 1 wherein said sheet has ahoneycomb of cells of dielectric material thereon between said strips.

. The invention as set forth in claim 5 wherein said cells arerectangular in shape and have their longer dimension substantiallyparallel to said strips.

7. The invention as set forth in claim 1 wherein said sheet is conicalin shape, said strips being disposed on the inner surface of the coneformed by said sheet.

8. The invention as set forth in claim 7 wherein said strips arecircular discs having their axes along the axis of said cone.

9. The invention as set forth in claim 8 wherein said sheet is aboutone-half wavelength in thickness at the frequency of the radiationtransmitted through said radome.

10. The invention as set forth in claim 1 wherein said sheet forms aconical surface and said strips are disposed along the axis of saidconical surface equally spaced from each other and directed so that theintersection of lines extending therefrom is at the apex of said conicalsurface.

11. The invention as set forth in claim 10 wherein said radome is anA-sandwich having inner and outer skins of dielectric material, saidouter skin being comprised of said sheet and wherein said stripscomprise the core of said sandwich.

12. The invention as set forth in claim 11 wherein said core includes ahoneycomb of rectangular cells of dielectric material disposed betweensaid strips, the long dimension of said cells being substantiallyparallel to said strips.

1. A radome comprising: a. a sheet of dielectric material, and b. an array of thin strips of dielectric material supported edgewise on said sheet with the edges of said strips in contact with said sheet, said strips being arranged in planes spaced from each other, said planes being parallel to the plane of incidence of at least one of two polarizations of radiation which are perpendicular to each other, said planes lying transverse to said sheet, said array providing anisotropy favoring said one polarization of radiation.
 2. The invention as set forth in claim 1 wherein said strips are parallel to each other.
 3. The invention as set forth in claim 2 wherein said strips each are at least a quarter wavelength in a direction along the plane of said one polarization of radiation and are a fraction of said quarter wavelength in thickness measured from side to side along their supported edges.
 4. The invention as set forth in claim 3 wherein the spacing of said strips from each other is equal and is from one-half to 1 1/2 of a wavelength of the frequency of the radiation to be transmitted through said radome.
 5. The invention as set forth in claim 1 wherein said sheet has a honeycomb of cells of dielectric material thereon between said strips.
 6. The invention as set forth in claim 5 wherein said cells are rectangular in shape and have their longer dimension substantially parallel to said strips.
 7. The invention as set forth in claim 1 wherein said sheet is conical in shape, said strips being disposed on the inner surface of the cone formed by said sheet.
 8. The invention as set forth in claim 7 wherein said strips are circular discs having their axes along the axis of said cone.
 9. The invention as set forth in claim 8 wherein said sheet is about one-half wavelength in thickness at the frequency of the radiation transmitted through said radome.
 10. The invention as set forth in claim 1 wherein said sheet forms a conical surface and said strips are disposed along the axis of said conical surface equally spaced from each other and directed so that the intersection of lines extending therefrom is at the apex of said conical surface.
 11. The invention as set forth in claim 10 wherein said radome is an A-sandwich having inner and outer skins of dielectric material, said outer skin being comprised of said sheet and wherein said strips comprise the core of said sandwich.
 12. The invention as set forth in claim 11 wherein said core includes a honeycomb of rectangular cells of dielectric material disposed between said strips, the long dimension of said cells being substantially parallel to said strips. 